Biaxially textured metal oxide buffer layers on metal substrates are potentially useful in electronic devices where an electronically active layer is deposited on the buffer layer. The electronically active layer may be a superconductor, a semiconductor, or a ferroelectric material.
For example, the next generation of superconducting wire to be used for power transmission lines will have a multi-layer composition. Such deposited conductor systems consist of a metal substrate, buffer layer, and a superconducting layer. The metal substrate, such as Ni, Ag, or Ni alloys, provides flexibility and support for the wire. Metal oxide buffer layers, such as cerium oxide (CeO.sub.2), or yttria-stabilized zirconia (YSZ), comprise the next layer and serve as chemical barriers between the metal substrate and the top layer, the high-temperature superconductor.
For a superconducting film to carry a high current, a certain degree of alignment between grains of the superconductor is required. Most preferably, the grains should be aligned both perpendicular to the plane of the substrate (c-axis oriented) and parallel to the plane of the substrate (a-b alignment). To achieve this alignment, high T.sub.C superconductors have generally been deposited on (100) oriented single-crystal oxide substrates. However, single-crystal substrates are generally too expensive and have poor mechanical properties. As such, single-crystal substrates are presently unsuitable as practical conductors.
A method to develop practical coated conductors is disclosed in U.S. Pat. No. 5,741,377 ('377) by Goyal et al. This method called RABiTs, short for rolling assisted biaxially textured substrates, uses roll-texturing of metal to form a metallic tape with a {100}&lt;001&gt; cubic structure. However, if the metal is nickel or a nickel alloy, a buffer layer between the metal substrate and the ceramic superconductor is necessary to prevent interdiffusion of the ceramic superconductor and the metal substrate and also to prevent the oxidation of nickel substrate during the deposition of the superconducting layer. Useful buffer layers include cerium oxide, yttrium stabilized zirconia (YSZ), strontium titanium oxide, rare-earth aluminates and various rare-earth oxides.
Conductors based on the RABiTs approach typically consist of a biaxially textured metal substrate, one or more buffer layers (usually oxides), and the superconducting compound YBCO or one of the Bi, Ti, or Hg superconductors. To achieve high critical current densities, it is important that the biaxial orientation be transferred from the substrate to the superconducting material. As stated, a biaxially textured metal substrate can be provided by the method disclosed in the '377 patent.
The purpose of the buffer layers is to transmit the biaxial texture of the metallic substrate to the superconductor and to prevent NiO formation and chemical interactions between the metal substrate and YBa.sub.2 Cu.sub.3 O.sub.7-.delta. (YBCO). The conventional processes that are currently being used to grow buffer layers on metal substrates and achieve this transfer of texture are vacuum processes such as pulsed laser deposition, sputtering, and electron beam evaporation. Researchers have recently used such techniques to grow biaxially textured YBCO films on metal substrate/buffer layer samples that have yielded critical current densities (J.sub.C) between 700,000 and 10.sup.6 A/cm.sup.2 at 77 K (A. Goyal, et al., "Materials Research Society Spring Meeting, San Francisco, Calif., 1996; X. D. Wu, et al., Appl. Phys. Lett. 67:2397, 1995). One drawback of such vacuum processes is the difficulty of coating long or irregularly shaped substrates, and the long deposition times and relatively high temperatures required.
Another purpose of the buffer layers is to prevent oxidation of the metal substrate (for example NiO, when using Ni). If the Ni begins to oxidize, the resulting NiO will likely to grow in the (111) orientation regardless of the orientation of the Ni (J. V. Cathcart, et al., J. Electrochem. Soc. 116:664, 1969). This (111) NiO orientation adversely affects the growth of biaxially textured layers and will be transferred, despite the substrate's original orientation, to the following layers.
A typical architecture is Ni/CeO.sub.2 /YSZ/CeO.sub.2 /YBCO. The CeO.sub.2 layers are kept thin to avoid cracking and the thicker YSZ layer provides chemical protection. The top layer of CeO.sub.2 is included because the lattice parameter of YSZ does not match that of YBCO very well. The difference is about 5%.
For producing high current YBCO conductors on {100}&lt;001&gt; textured Ni substrates, high quality buffer layers are necessary. Buffer layers such as CeO.sub.2 and YSZ have previously been deposited using pulsed laser ablation, e-beam evaporation, and sputtering. In addition, solution techniques have been used to deposit films of rare-earth aluminates on biaxially textured nickel substrates. However, the rare-earth aluminates had c-axis alignment but have frequently given a mixture of two epitaxies (100)[001] and (100)[011]. This is a structure believed to be unsuitable for growth of high critical current films.
It has been demonstrated that RE.sub.2 O.sub.3 (rare earth oxides) can be grown epitaxially on textured Ni substrates by both reactive evaporation and sol-gel processing techniques. However, the process window for growing RE.sub.2 O.sub.3 films are very narrow. Additionally, some of the rare earth oxides go through a cube to monoclinic phase transition.